The purpose of the proposed study is to evaluate the distinct beneficial effects of aerobic exercise and resistance exercise for inactive older (aged 65+) breast cancer survivors (BCS) at least two years post- treatment. We propose to conduct a 12-month randomized controlled trial with three groups: 1) a progressive, supervised aerobic exercise program, 2) a progressive, supervised resistance exercise program, and 3.) a control condition consisting of flexibility/relaxation sessions. The primary specific aims are to test the following hypotheses: 1) Aerobic exercise will decrease fatigue, increase aerobic capacity, and improve functioning and health outcomes (physical functioning, physical health, and mental health), compared to a control group, and group differences will remain at 6-month follow-up;and 2) Resistance exercise will decrease fatigue, increase muscle strength, and improve functioning and health outcomes (physical functioning, physical health, and mental health), compared to a control group, and group differences will remain at 6-month follow-up. Secondary aims will evaluate the differences between outcomes in the aerobic and resistance groups, test mediator effects of fatigue and muscle strength, and compare the three groups in terms of muscle mass, body fat, and biomarkers for risk of cancer recurrence. The proposed study is significant because it will be one of the first to study the effects of any exercise in older BCS, and the first to compare the relative benefits of aerobic and resistance exercise in BCS of any age. The long-term goal of the proposed research is to develop interventions that target the specific needs of older breast cancer survivors, with a maximum benefit for each individual's time and effort. This application addresses the 2006 strategic goal of the National Cancer Institute to invest in intervention research focusing on long-term health in cancer survivors, especially in the underserved population of older adults. This study is relevant to public health because the knowledge gained will guide the development of evidence-based interventions to improve the health of a large segment of the BCS population. Approximately 85% of women who receive a first diagnosis of breast cancer are aged 50 and over, thus older women constitute the largest group of BCS. Today, more than 1.2 million women aged 65+ are BCS, with original cancer diagnosis from 1 to 27 years ago, a figure that will increase dramatically as the U.S. "baby boom"population ages. Even so, studies of older women comprise only a small proportion of research, especially exercise research, on improving the lives of cancer survivors.